The diketopyrrolopyrrole family of compounds and their pigmentary properties are well known. The pigmentary diketopyrrolopyrroles include unsubstituted 1,4-diketopyrrolopyrrole as well as various substituted diketopyrrolopyrroles, including those substituted by isocyclic or heterocyclic aromatic radicals. They are suitable for pigmenting organic materials of high molecular weight.
It is known in the pigments art that substituted diketopyrrolopyrrole pigments can be prepared by the reaction of a mole of a disuccinate with two moles of an aromatic nitrile or one mole each of two different aromatic nitriles. U.S. Pat. No. 4,579,949 describes reaction of a disuccinate with aromatic nitriles in an organic solvent and in the presence of a strong base at elevated temperature, and subsequently protolyzing the resultant salt. The product of such process, known as crude diketopyrrolopyrrole, generally has a medium to large particle size, which size has been shown to be unsuitable for certain end uses, such as in automotive coatings, requiring a high degree of transparency. Many automotive styles with metallic-effects are created by the use of a combination of aluminum or mica flakes and uniformly small particle size pigments. It therefore becomes necessary to further process the larger particle size crude pigments to develop the requisite pigmentary properties, such as smaller particle size, particle shape, polymorphic phase and tinctorial strength.
Particle size manipulation has thus become a significant art in pigment technology. Highly desirable pigments are traditionally produced by subjecting the crude pigments to a variety of pigment finishing methods, also called pigment conditioning steps, the purpose of which is to create pigments of defined particle size with a narrower particle size distribution, preferably in a single homogeneous crystal phase. In the case of diketopyrrolopyrroles, the crude form is commonly convened to a useable pigmentary form by milling it with large quantities of inorganic salt followed by extraction of the resulting mill powder, or by dissolving the pigment in large quantities of sulfuric acid and drowning the solution in water (known as acid pasting). These multistep procedures generally require a diversity of operations conducted at elevated temperatures in acidic environments; therefore, simpler and more economical procedures for preparing pigmentary diketopyrrolopyrroles are highly desired in the industry.
Work has been carried out in the art to prepare smaller particulate forms of diketopyrrolopyrrole pigments. For example, U.S. Pat. No. 5,502,208 relates that certain cyano-substituted diketopyrrolopyrrole pigments can be prepared in finely particulate form by carrying out the protonation step in water and/or an alcohol in the presence of an acid in an amount sufficient to keep the pH less than 9, and a temperature of greater than 90.degree. C. Inclusion of particle growth inhibitors during the synthesis of diketopyrrolopyrroles is not mentioned.
It is known from U.S. Pat. Nos. 5,457,203 and 5,424,429 that certain phthalimidomethylquinacridone compounds are useful as a growth inhibitor in the oxidation reaction of a dihydroquinacridone to form a quinacridone. The conversion of dihydroquinacridone to quinacridone is performed using a mild oxidizing agent in an alkaline alcoholic medium. It is proposed that under these oxidation conditions the phthalimidomethyl quinacridone partially ring opens to an alkali metal salt of the carboxy carbonamide.
It is, however, surprising that the synthesis of a diketopyrrolopyrrole proceeds smoothly in the presence of phthalimidomethylquinacridone to generate a uniformly small particle size pigment. The phthalimidomethylquinacridone molecule, which is structurally and chemically dissimilar to a diketopyrrolopyrrole, unexpectedly acts as a particle size adjusting template for the diketopyrrolopyrrole. Also surprisingly, the phthalimidomethylquinacridone derivative has no detrimental effect on either the color saturation or the chemical yield of the synthesized diketopyrrolopyrrole pigment. While the conversion of dihydroquinacridone to quinacridone, as described in U.S. Pat. Nos. 5,457,203 and 5,424,429, involves only aromatization of the pentacyclic heterocycle, in the case of diketopyrrolopyrroles, the addition of the dienolate of the succinate to aromatic nitrites involves intramolecular ring closure. The reaction mechanisms for the synthesis of the two types of pigments are entirely different.
The primary object of the present invention is to prepare useful pigmentary diketopyrrolopyrroles directly during synthesis, without the need for particle size reducing aftertreatments. The synthetic method of the present invention is highly desirable as it avoids multistep pigment finishing procedures which create a burden on the environment. Further, this route to the synthesis of small particle size pigments, when adding growth inhibitors during the addition of nitrites to the succinate in the presence of a strong deprotonating agent, is of great economic significance. Other objects will become apparent upon reference to the more detailed description.